When a 14-year-old Mattapan boy allegedly shot and killed his brother last week, a collective question rose throughout the horrified streets of Boston.

“Where did he get the gun?”

It is a question that is dogging Boston Police Commissioner William B. Evans and Mayor Martin J. Walsh, both of whom are preparing to relaunch the city’s gun buyback program, which encourages residents to turn in their unwanted firearms.

But as Walsh and Evans work for a solution, researchers and crime fight advocates across the city and nation are raising doubts about whether gun buybacks are effective in curbing street crime or just a panacea for frustrated members of the public.